That's most likely an error with your email provider. Contact them if they didn't fix it themselves in a few hours.
Also it can be caused by certain things on your PC, like:
- bad connection with Internet
- antivirus add-on checking the mail
- firewall settings
- changes in the Outlook settings you made
- malware interfering
If that doesn't ring a bell (like: yes, I installed new antivirus just an hour ago) it most likely is an error with them, so I would only try searching for those causes if their customer service says everything is OK with them.
I am using Outlook 2010 from the last 10 months in my office. Currently, I have to send the emails but the outlook is showing the message pop up of error 0x8004210a. So, I am looking for the solution.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic